Ca promotes photosynthesis
Our results indicated Ca deficiency significantly inhibited P.
massoniana photosynthetic process, while sufficient Ca supply markedly
facilitated photosynthesis (Figure 1). Consistently, proteomic analysis
revealed the down-regulation of a plenty of photosynthesis related
proteins under low Ca level and the recovered and even increased
expression of these proteins under high Ca level, including both light
reaction and dark reaction related proteins (Table 1). PPI network
analysis indicated photosynthesis is an important function module of Ca
influenced physiological process (Figure 5). A previous review has well
characterized the role of Ca in plant
photosynthesis.15 Our
study further consolidated the important role of Ca in photosynthesis
and demonstrated the mechanism of Ca regulated photosynthesis inP. massoniana .
PsbP family protein, one of the key components of photosystem II in
higher plants, can enhance oxygen evolution rate at physiological Ca and
chloride
concentration.32 The
high Ca treatment activated the up-regulation of Mog1/PsbP/DUF1795-like
photosystem II reaction center PsbP family protein (spot 14), suggesting
photosynthetic oxygen evolution may be accelerated by adequate Ca
supply. The changed expression abundance of chlorophyll A/B binding
protein (spot 15), plastid high chlorophyll fluorescence 136 precursor
(spot 19), flavoprotein WrbA-like (spot 23) and oxygen-evolving enhancer
protein 1 (spot 25) imply other light reaction processes could also be
affected by Ca level.
Ribose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is the
most abundant cellular soluble protein in plant, catalyzes the initial
step of CO2 fixation and is a rate-limiting enzyme for
CO2assimilation.33 The
down regulation of a variety of RuBisCO related proteins (spots 13, 17,
20, 22, 24, 26, 27, 28) indicated CO2 fixation and
assimilation potentially be hindered under Ca deficiency condition. High
Ca treatment dramatically reversed this situation; significant
up-regulation of these proteins were observed, suggesting the promoting
role of Ca in CO2 assimilation. Chloroplast stroma
located carbonic anhydrase (CA), the second richest plant cellular
protein apart from RuBisCO, catalyzes the inter-conversion between
CO2 and HCO3- and
enhance the delivery of CO2 to
RuBisCO.34 It is
generally up-regulated at moderate stress severity to respond stress
though partially compensating for decreased
CO2-conducting
aquaporin.35Accordingly, we observe the interaction among CA1, Rubisco large
subnunit (RBCL) and Rubisco small subnunit (RBSL) in the constructed PPI
network (Figure 5). In the presented study, the up regulated CA1 (spot
11) suggests Ca deficiency bring severe CO2 fixation
obstruct in P. massoniana seedlings. High Ca treatment lead to
the down-regulation of CA1, which is a naturally result of high Ca
restored normal photosynthetic carbohydrate fixation.
Glutamate-glyoxylate aminotransferase (GGAT) is a critical enzyme
engaged in photorespiration, which is an energy and organic carbohydrate
consumption process in
plant.36 Ca deficiency
possibly increased photorespiration while inhibited photosynthesis as
significantly up-regulation of GGAT 2-like protein were observed at low
Ca level. High Ca may lower photorespiration by down-regulating GGAT
related proteins.
Collectively, Ca deficiency impaired the photosynthesis process inP. massoniana seedlings. Adequate Ca supply improved
photosynthesis through promoting CO2 fixation,
regulating light reaction and inhibiting photorespiration processes.